I copied my Excel cells into Word as a picture. How can I remove the cell border lines in this process?

I assume you are using a screen clipping tool to copy the Excel cells into Word. I would recommend either copying the cells (dragging to select) and pasting as a table into Word, or embedding the spreadsheet into Word. Method one works perfectly for basic Excel content and allows you to further format it to suit the structure and styling of your Word document. Method two allows complicated Excel content such as graphs to appear in Word without making screen clippings. It also can be used to preserve all formulas in the embedded sheet so that you can access them directly in Word. Either of these methods should help your Excel cells appear correctly in your Word document.